April 23, 2019 : A New Study Suggests That The Cold Stratospheric Polar Vortices On Titan May Last 22 Earth Years Or 3 Titanian Seasons
A new study entitled « Seasonal Evolution of Titan's Stratosphere During the Cassini Mission », recently published in AGU's journal Geophysical Research Letters and led by Nick Teanby who is a planetary scientist at the University of Bristol in the United Kingdom, reveals that the polar vortices observed on Saturn's largest moon Titan may last 22 Earth years or 3 Titanian seasons. During the long Cassini mission, a giant ethane cloud engulfing the north polar region had been clearly observed during the Winter season in the northern hemisphere and a vortex or a cyclone developing over the south polar region during the Autumn season in the southern hemisphere had been clearly identified as well. The new analyses based on data acquired from the Cassini orbiter and on atmospheric science developed on our planet to understand the logic of seasonal changes on the Opaque Moon reveal the cooling effects of trace gases found in the high-altitude polar vortices. A parallel can be drawn between the polar vortices of Titan and the polar vortices of our planet even if the environmental temperature of the Hazy Moon is much lower than that of the Earth.
Titan's atmosphere is particularly complex with its haze and its hydrological cycle dominated by methane. There are lakes, seas, rivers, evaporation processes, condensation processes and precipitation processes on the giant moon of Saturn. The stratosphere of the Orange Moon which evolves above the troposphere is composed of cooler layers in its lower part and of warmer layers in its upper part. The polar vortices of that enigmatic world can be found in the stratosphere. Those systems of cold air evolving above the polar area clearly appear during the Winter season. The particularly cold environment that can take shape in North America during the Winter season is due to that type of atmospheric system. On the Blue Planet, the vortex tends to dissipate in the Spring season. However, on Saturn's largest moon, the polar vortex identified during the Winter season of the northern hemisphere was still present at the time of the Summer Solstice in the northern hemisphere. Therefore, one can conclude that the vortex may last around 22 Earth years or 3 Titanian seasons.
The planetologists had previously shown that the presence of the polar vortex on the Hazy Moon explained the enrichment of trace gases in its stratosphere. Those trace gases are closely linked to the surprisingly intense cold identified in the polar vortex of the southern hemisphere at the beginning of the Winter season in that area. The new research work suggests that the combination of cooling engendered by trace gases and of warming produced by sinking air breaks the Winter season of the Opaque Moon into two phases. Nick Teanby pointed out : « Earth cools in winter due to lack of sunlight over the poles, but you don't get this added effect from extra gases, whereas on Titan you've got these weird gases in there that's making the process even more extreme than it would be otherwise. » The connection between the trace gases and the polar vortex had been well studied by Nick Teanby and his collaborators. The new research work represents an in-depth study of seasonal variation in the temperature and composition of the stratosphere of that enigmatic world on the basis of infrared mapping data acquired from the Cassini orbiter during its remarkably long mission in the System of Saturn from 2004 to 2017.
Thanks to the Cassini spacecraft, we've had the opportunity to observe the high-altitude polar vortex of the northern hemisphere when the north pole was tilted away from our star and to observe the high-altitude polar vortex of the southern hemisphere during the Autumn season in the southern hemisphere after the Equinox of 2009. Each season is particularly long on the giant moon since a Titanian year represents about 29.5 Earth years. Claire Newman, a scientist who is an expert in planetary atmospheres at Aeolis Research and who is not part of the research work, pointed out : « This is the first time one paper has gone into the whole of the Cassini dataset, covering almost half of the Titan year, and looked at how northern and southern polar vortex evolution might differ. » She added : « I work on atmospheric models and we rely on these kinds of observations to understand how correctly our models are capturing what is going on on Titan itself. » The planetologists hope to obtain enough data, in the future, to be in a position to apply the atmospheric models of our planet to Saturn's largest moon and to anticipate the evolution of the Titanian atmosphere in terms of composition or in terms of dynamics.
The study of Titan's atmosphere may also allow us to improve our climate models of the Blue Planet and to better understand our own atmosphere or climate as Nick Teanby explained. He advanced : « Why it's so interesting is that Titan is like a mini Earth with a really exotic and cold atmosphere that we can use to test climate models and things like that. » He added : « That's the big picture to why we bothered, but I guess the real motivation is just that it's really cool to try and figure this stuff out. » Like the Earth, Titan is governed by seasons since the obliquity of the Opaque Moon is a little bit higher than the obliquity of the Blue Planet. The Cassini spacecraft entered the Saturn System in 2004 when the northern hemisphere of the Orange Moon was experiencing the long Winter season and its targeted crash against Saturn occurred in 2017 at the beginning of the Summer season in the northern hemisphere. At the beginning of the Cassini-Huygens mission in the Saturn System, a polar vortex was present from the north pole to about 45 degrees north latitude, a latitude which can be found in the south of France for instance.
The vortex found above the north polar region in Winter represents a large cap of cold air and low pressure twisting in the direction of the rotation of the planetary body or moon. A clear separation between strong westerly jetstreams encircling the polar area and containing the cold and warm air from the equator is generated. Jetstream barriers prevent mixing of air masses so that the vortex remains cold and keeps its particular compounds or exotic chemicals. On our planet, the geographical limit of that impressive type of atmospheric system is found at approximately 60 degrees latitude which corresponds to the south of Alaska, the south of Sweden or the south of Norway. Regions in lower latitudes are likely to encounter the vortex which was the case in January 2019 when the circling jetstream lost some strength and meandered. The data captured by the Cassini orbiter have shown that the northern polar vortex had persisted through the Equinox and had vanished at the start of the Summer period in the northern hemisphere of Titan. That vortex had lasted longer than expected.
In parallel, a vortex started to develop over the south polar region during the Autumn season in the southern hemisphere. That vortex observed shortly after the Equinox appeared surprisingly colder than the polar vortex clearly observed in the Winter season in the northern hemisphere. The new study suggests that the difference may be related to an early Winter extra-cold phase generated by the chemistry of the haze or the atmosphere rather than fundamental differences between the polar areas. Titan's atmosphere is dominated by nitrogen like the atmosphere of our planet but the second most abundant gas on Titan is methane representing between 1 and 2 percent of the composition of the exotic atmosphere. A complex haze takes shape and develops under the influence of UV light from the Sun. The particular chemistry of hydrocarbons or organics of Titan's atmosphere may strengthen the polar vortex. In the upper atmosphere, molecules like cyanide, ethylene, ethane or larger hydrocarbons or organics can take shape via the interactions between solar radiations, the energy from Saturn's magnetic field and the particles, ions, radicals or molecules. The haze of the Orange Moon is closely related to that type of photochemistry.
Thanks to the Cassini orbiter, we have identified an enrichment of those trace gases above the Winter poles of Saturn's largest moon and the new study has determined that the enrichment is higher at the beginning of Winter, when the polar area, experiencing the Winter season, is also colder. The temperature difference between the equatorial area and the dark Winter pole engenders the formation of the polar vortex on Titan or even on Earth. Logically, in the polar area, cold air goes down and drags the upper part of the atmosphere downward during the Winter season. The process of mixing with the colder mid-layers of the atmosphere of the Orange Moon during the downward migration of trace gases leads to condensation processes or to liquid or solid clouds. There can be rain of hydrocarbons on Titan. The trace gases which have condensed act like a sink, accelerating the downward migration of more trace gases from the upper part of the atmosphere where they take shape.
The cold layers of the atmosphere of the Opaque Moon are made colder by trace gases because those trace gases emit infrared radiations or heat. If they glow, their level of energy decreases so that the atmosphere cools since the energy is radiated into outer space. The new research work suggests that the now even colder air goes down faster, in an extremely cold feedback cycle. Nick Teanby argued : « That's all happening at the start of winter, so the start of winter is really, really cold. » Over time, the pressure rise engendered by all that sinking mass of air produces its own heat which reduces the decrease in the environmental temperature or simply increases the environmental temperature in that feedback cycle. The researchers believe that those processes lead to the development of two distinct phases in the Winter period on Titan. Nick Teanby pointed out : « As you go deeper into winter and the circulation's more developed, you get an opposite effect, where you start to warm the stratosphere due to this compression of the air as it's sinking. So, there's these two phases to winter that are quite strange. We're not totally sure that's what's happening, but that's our theory at the minute. » One can imagine that the high-altitude polar vortex of the southern hemisphere of Titan, currently experiencing the Winter season, has grown since the end of the journey of the Cassini orbiter.
The image above reveals the high-altitude vortex located over the south polar region of Saturn's largest moon Titan during the Autumn season in the southern hemisphere. That vortex was expected to continue to develop at the time of the observation. The view was obtained on August 31, 2012 with the Narrow-Angle Camera of the Cassini spacecraft on the basis of a spectral filter sensitive to wavelengths of near-infrared radiation centered at 938 nanometers. Image credit: NASA/JPL-Caltech/Space Science Institute.
- To get further information on that news, go to: https://blogs.agu.org/geospace/2019/04/11/extended-winter-polar-vortices-chill-saturns-strangely-familiar-moon-titan and https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018GL081401.